A conventional color image projection system granted in U.S. Pat. No. 4,368,963 as illustrated in FIG. 4.
This color image projection system includes a large screen 1, three lenses 2, three identical liquid crystal panels 3, three color filters 4, 5 and 6, respectively of the colors red, green and blue, three white light sources 7 and three reflectors 8.
In this projection system, light from the light sources 7 passes through the corresponding filters 4, 5 and 6 and a liquid crystal (LC) panel 3, resulting in its incidence into the respective lenses 2.
Since the respective lenses 2 are focused in the same place onto the screen 1, the mixture of the three colored composites projected on the screen via the lenses 2 creates a multicolor image.
For example, if the composite portions of an image or picture must be in the color red, the light intensity is modulated by connecting the variable frequency and/or amplitude voltage source 31 to the LC panel 3 and varying the applied voltage and the frequency of the applied volatge.
Behind the LC panel 3 is disposed the red filter 4, which is made of red transparent glass or plastic. The color filters 4, 5 and 6 could alternatively be disposed in front of the LC panel 3 or even in front of the lenses 2.
Behind the light sources 7, there are reflectors 8 for improving light efficiency. Light from each of the sources 7 passes through a respective filter and through a liquid crystal panel 3. Each respective lens magnifies and projects all of the energized port ions of the liquid crystal panel onto the screen. By such manner, the mixture of the three colored composites on the screen creates a multicolor image or picture with high resolution.
However, in the case of a conventional projection system, due to the close distance between the sources 7 and the LC panels 4, 5 and 6, in order to raise the brightness, a cooling device is needed because heat generated from the source 7 has an influence over the LCD characteristics. However, difficulties in the technology of this device result in a limitation in the light intensity. Accordingly, the light intensity cannot be suitably modulated, so that there are difficulties in the modulation of the brightness. Thus, a conventional system has a definite disadvantage of reducing the brightness on the screen.